Yermak Timofeyevich (Russian: Ермак Тимофеевич, IPA: [jɪrˈmak tʲɪmɐˈfʲejɪvʲɪt͡ɕ]; c. 1532 – 5 or 6 August 1585) was a Cossack ataman, a zemleprokhodets, who started the Russian conquest of Siberia during the reign of the Russian Tsar Ivan the Terrible. He is today a hero in Russian folklore and myths.

Russian interests in the fur trade fueled their desire to expand east into Siberia. The Tatar Khanate of Kazan established by Ulugh Muhammad was the best entryway into Siberia. In 1552, Ivan the Terrible's modernized army toppled the khanate. After the takeover of Kazan, the tsar looked to the powerful and affluent Stroganov merchant family to spearhead the eastward expansion. In the late 1570s, the Stroganovs recruited Cossack fighters to invade Asia on behalf of the tsar. These Cossacks elected Yermak as the leader of their armed forces, and in 1582 Yermak set out with an army of 840 to attack the Khanate of Sibir.

On October 26, 1582, Yermak and his soldiers overthrew Kuchum Khan's Tatar empire at Qashliq in a battle that marked the "conquest of Siberia". Yermak remained in Siberia and continued his struggle against the Tatars until 1584, when a raid organized by Kuchum Khan ambushed and killed him and his party.

The specifics of Yermak's life, such as his appearance, background, and dates of events, remain points of controversy for historians, because the texts that document his life are not reliable. However, his life and conquests had a profound influence on Siberian relations, sparking Russian interest in the region and establishing the Tsardom of Russia as an imperial power east of the Urals.

There is less information about Yermak than most other notable explorers and historical figures. Much of what we know about Yermak is derived from folklore and legend. There are no contemporary descriptions of Yermak and all portraits are merely estimations. One of the Siberian chronicles, the Remezov Chronicle, written more than one hundred years after Yermak's death describes him as "flat-faced, black of beard with curly hair, of medium stature and thick-set and broad-shouldered," but even this detailed account is not reliable because the narrator had never seen Yermak.

In addition to his physical features' being unknown, the details of Yermak's life and the circumstances leading up to his excursion into Siberia are obscure. Russian writer Valentin Rasputin laments the lack of information that we have about Yermak considering the vast scope of his contributions to Russian society.

Knowledge of Yermak's upbringing and voyages pales in comparison to that of other explorers. Historians encounter serious difficulties when attempting to piece together the specifics of Yermak's life and exploits because the two key, primary sources about Yermak may be biased or inaccurate. These sources are the Stroganov Chronicle and the Sinodnik. The Stroganov Chronicle was commissioned by the Stroganov family itself, therefore it exaggerates the family's involvement in the conquest of Siberia. The Sinodnik is an account of Yermak's campaign written forty years after his death by the archbishop of Tobolsk, Cyprian (Kipriyan). The text was formed based on oral tradition and memories of his expedition but almost certainly was affected by the archbishop's desire to canonize Yermak. The combination of forgotten details over time and the embellishment or omission of facts in order for Yermak to be accepted as a saint suggests that the Sinodik could be erroneous. Though Cyprian failed to canonize Yermak, he made an effort to immortalize the warrior, who he considered being the "Grand Inquisitor" of Siberia.

These documents, along with the various others that chronicle Yermak's expeditions, are filled with contradictions that make the truth about Yermak's life difficult to discern. While the sources that exist on Yermak are fallible, those accounts, along with folklore and legend, are all that historians have to base their knowledge on; therefore, they are widely accepted and considered to reflect the truth.